In a Coriolis flowmeter, a tube through which a fluid to be measured flows is supported at one end or both ends thereof, and vibration is applied to a portion of the tube around the supporting point in a direction vertical to the flowing direction of the tube (hereinafter, a tube to which vibration is applied is referred to as a flow tube). The Coriolis flowmeter is a mass flowmeter, which utilizes the fact that the Coriolis forces applied to the flow tube when vibration is thus applied thereto, are proportional to a mass flow rate. The Coriolis flowmeter, which is well known, is roughly classified into two types in terms of flow tube structure: a straight tube type and a bent tube type.
In a Coriolis flowmeter of the straight tube type, when vibration is applied to a straight tube, whose both ends are supported, in a direction vertical to the straight-tube center portion axis, a difference in displacement due to the Coriolis forces is generated between the support portions and the central portion of the straight tube, (that is, a phase difference signal is obtained), and, based on this phase difference signal, the mass flow rate is detected. The straight tube type Coriolis flowmeter, thus constructed has a simple, compact, and solid structure. On the other hand, there arises a problem in that it is difficult to achieve high detection sensitivity.
In contrast, the bent tube type Coriolis flowmeter is superior to the straight tube type Coriolis flowmeter from a view point that it allows selection of a shape for effectively obtaining the Coriolis forces. In fact, it is capable of performing mass flow rate detection with high sensitivity. Known examples of the bent tube type Coriolis flowmeter include one equipped with a single flow tube (see, for example, JP 04-55250 A), one equipped with two flow tubes arranged in parallel (see, for example, Japanese Patent 2939242), and one equipped with a single flow tube in a looped state (see, for example, JP 05-69453 A).
Incidentally, as a driving means for driving the flow tube, a combination of a coil and a magnet is generally employed. Regarding the mounting of the coil and the magnet, it is desirable to mount them at positions not offset with respect to the vibrating direction of the flow tube from the viewpoint of minimizing the positional deviation between the coil and the magnet. In view of this, Japanese Patent 2939242 discloses a construction in which two flow tubes arranged in parallel are mounted so as to hold a coil and a magnet between them. Thus, a design is adopted in which the distance between the two flow tubes opposed to each other is at least large enough to enable the coil and the magnet to be held therebetween.
In the case of a Coriolis flowmeter in which two flow tubes respectively exist in planes parallel to each other and which exhibits a large caliber or high flow tube rigidity, it is necessary to enhance the power of the driving means, so that it is necessary to hold a large driving means between the two flow tubes. Thus, a design is adopted in which the distance between the flow tubes is of necessity large even at the fixing end portions constituting the roots of the flow tubes.
However, an increase in the above-mentioned distance at the fixing end portions involves the following problem: it leads to a deficiency in rigidity at the fixing end portions, with the result that vibration leakage is likely to occur (The flow tubes undergo bending vibration to cause vibration leakage).
On the other hand, in the construction as disclosed in JP 05-69453 B, in which a single flow tube is looped, another problem is involved: as shown in FIGS. 11 and 12, it is necessary for a bent tube portion 101 to exist between a first curved tube portion 102 and a second curved tube portion 103. If such a sharp bending is to be effected, the manufacture is rather difficult and, further, there is a problem in terms of the resistance to pressure of the tubes.